A Group III nitride semiconductor material such as gallium indium nitride (GaInN) has been conventionally used for a light-emitting layer of a short-wavelength light-emitting diode (abbr.: LED) of, for example, white or blue or a laser diode (abbr.: LD) (see, for example, JP-B No. 55-3834). Aluminum gallium nitride (AlGaN) has been used as a material for a light-emitting layer of a near-ultra violet LED or an ultra violet LED (see, for example, JP-A No. 2001-60719).
One type of the conventional white LEDs comprises chip or lamp shaped red, green, and blue LEDs respectively emitting color lights of red (R), green (G), and blue (B), which are light's three primary colors. The numbers of red, green, and blue LEDs used respectively correspond to the relative emission intensity ratios. The LEDs are integrated and arranged on the same substrate, and, as a whole, the mixture of red, blue, and green provides emission of white light (see, for example, JP-A Nos. 6-314824, 7-7223, 7-15044, 7-235624, 7-288341, 7-283438, and 7-335942). This type of white LED may be referred to as an array-type (module) white LED.
Another type of conventional white LED includes light-emitting layers which are separately provided on one substrate, each emit red, green, and blue lights, and are made of, for example, a Group III nitride semiconductor (see, for example, JP-A Nos. 6-53549 and 7-183576). The white LED may be referred to as an RGB-type white LED which emits white color by mixing lights corresponding to the light's three primary colors (red (R), green (G), and blue (B)) emitted from respective light-emitting layers.
Still another type of conventional white LED is a white LED having light-emitting layers which respectively emit light in a relation of complementary colors and are provided on a single substrate. For example, in the white LED, a Group III nitride semiconductor light-emitting layer emitting blue light and a light-emitting layer emitting yellow light are respectively provided on the same substrate, and two lights which are emitted from the respective light-emitting layers different in color (for example, blue and yellow) and wavelength are mixed to emit white color (see, for example, JP-A No. 2001-257379). The white LED may be referred to as a complementary color type white LED utilizing the fact that when two lights of different colors (different wavelengths) in a complementary relationship are mixed, the mixture is visually perceived as a white light.
In addition to the above three types of LED, there is an LED which utilizes light emitted from a Group III nitride semiconductor light-emitting layer to excite a phosphor emitting fluorescence with a wavelength different from the light emitted from the light-emitting layer, and, thus, to convert the wavelength of the emitted light (see, for example, JP-A No. 7-99345). For example, the LED may be referred to as a fluorescent-type white LED which utilizes blue light or ultra violet light emitted from a Group III nitride semiconductor light-emitting layer to excite a phosphor, and, thus, to emit white light (see, for example, Japanese Patent Nos. 2900928, 3724490 and 3724498). As the phosphor excited by blue light or ultra violet light to provide white light, yttrium aluminum garnet (Y3Al5O12) and the like are used (see, for example, Japanese Patent Nos. 2927279, 3503139 and 3700502).
However, in the array-type white LED, for example, a plane area of the light-emitting layers respectively emitting red, green, or blue light is much smaller than the installation plane area required for integration and arrangement of the chip or lamp shaped red, green, or blue LEDs (see JP-A Nos. 6-314824 and 7-15044 to 7-335942).
That is, since the plane area occupied by the light-emitting layers providing light emission is extremely smaller than the plane area required for installation of the lamps, it is disadvantageous in obtaining a light-emitting device with high luminous intensity (lumen/area).
For example, when a substantially square LED chip with a side of 0.3 mm is surrounded by a resin and has a shell-shaped vertical cross section and a circular horizontal cross section to form a lamp of a typical shape, the outer diameter (the diameter) of the lamp is generally 3 to 5 mm (see paragraph (0007) of JP-A No. 6-314824). Thus, for example, in a lamp with an outer diameter of 5 mm, even when a light-emitting layer exists on the entire plane surface of a chip, the plane area is 0.09 mm2, which is much smaller than the plane area of the lamp (about 20 mm2). Thus, it is never advantageous in obtaining a light-emitting element with higher luminous intensity.
In the RGB-type white LED, the light-emitting layers which can respectively emit red (R), green (B), and blue (B) light are required to be separately provided. In addition to the requirement of providing a plurality of light-emitting layers, a clad layer or the like accompanying the light-emitting layer are required to be provided for each of the light-emitting layers in order to confine carriers (electrons and electron holes) in the light-emitting layer and confine light emission caused by radiative recombination of the carriers. Thus, a plurality of light-emitting layers are required to be provided on a single substrate, and, more preferably, the light-emitting layers and the clad layers or the like hetero-junctioned with the respective light-emitting layers are required to be provided. Thus, the process for forming the RGB-type white LED is complex and redundant. In this case, p-type and n-type electrodes are required to be provided for each light-emitting layer emitting different color light. Since the electrodes need to be provided in the clad layer of the conductivity type corresponding to each of the electrodes and the like, the light-emitting layer is scraped and removed, leading to deterioration of the luminous intensity of each emission.
Also in the complementary color type white LED, two or more light-emitting layers are required to be provided in order to emit lights of colors in a complementary relationship. In order to obtain light emission with higher luminous intensity, like the case of the RGB-type white LED, clad layers of the light-emitting layers are required to be joined to form a light-emitting part having a single hetero—(abbr.: SH) or double hetero—(abbr.: DH) junction structure. Thus, when a white LED of the complementary color type is formed, a complex and redundant process is required like the case of the RGB-type white LED.
Also when LEDs emitting light of different colors in a complementary relationship, such as blue light and yellow light, are closely arranged to constitute a white LED (see “Wide-gap semiconductor Opto and electronic device” (Mar. 31, 2006, Morikita Publishing Co., Ltd., first impression of the first edition), pp. 173-174), the total plane area of the light-emitting layer emitting blue or yellow light is smaller than the plane area required for arranging the LED, and therefore, it is not necessarily advantageous to obtain a light-emitting device with high luminous intensity.
In addition, in the complementary color type white LED, there is a problem that the color tone of resulting white light is slightly changed, depending on the wavelength of light of two colors in a complementary relationship that are mixed with each other to obtain the white light. In the complementary color type white LED, colors of at most two lights with different wavelengths are usually mixed, and therefore, in any case, it is technically difficult to stably obtain a white LED exhibiting high and stable color rendering properties.
In the fluorescent-type white LED, in order to stably obtain white light with a fixed color tone by excitation of the phosphor, the wavelength of the light emitted from the light-emitting layer, which acts as an excitation light, should be kept constant with good reproducibility, and thus it is technically difficult. Further, the composition of, for example, Y3Al5O12 used as a phosphor and containing a rare-earth element should be artificially and finely adjusted according to variation in the wavelength of emission from the light-emitting layers.
The present invention has been proposed in view of the above circumstances and an object thereof is to provide a Group III nitride semiconductor light-emitting device which has a simple structure that can be easily formed, can enhance luminous intensity, can obtain high and stable color rendering properties, and does not need fine adjustment of a composition of a phosphor.